This invention relates to acoustical wave flowmeters for measuring the rate of flow of the fluid along a confined path.
Acoustical wave flow meters are know which are employed to measure the velocity of a fluid moving in a confined volume. In a typical apparatus of this type, e.g. that disclosed in U.S. Pat. No. 3,109,112 issued Oct. 29, 1963, a pair of transducers capable of generating and receiving compressional waves in the audible or ultrasonic frequency range are placed inside an enclosure confining the fluid and defining a flow path therefore, with the transducers inserted directly in the flow path of the fluid. The transducers are alternately energized in a transmit and receive mode so that compressional waves are generated in the flowing fluid by a first transducer and received by the other transducer, the compressional waves traveling alternately upstream and downstream. By measuring the phase difference between the transmitted and received waves in both directions, the velocity of the fluid along the flow path may be determined.
In a modification of this technique, as expressly disclosed in the above referenced U.S. Patent, each transducer may comprise a compound annular ring transducer having both a transmit and a receive section specifically configured to minimize the adverse affects of thermal gradients in the flowing fluid. In another modification of the above known technique, four transducers are employed: two receive transducers spaced by the fixed distance L, and two transmit transducers positioned on the downstream and upstream side of the receiving pair. Other arrangements employing multiple transducers are also known.
All of the foregoing known flow meter configurations require that the transducers be physically located in either the flow path of the fluid whose velocity is to be measured or in cavities introduced into the walls of the fluid conduit. With either arrangement the normal flow of the fluid is substantially altered in passing the transducer sites. Because of this introduced turbulence, the accuracy of the measurements obtained with such systems suffers accordingly. In addition, contaminant materials present in the fluid, such as sedimentary particles or the like, become attached to the transducer surfaces, thereby impairing the transmitting and receiving characteristics thereof. In addition, in some ranges of pressures and fluid velocities, the force exerted by the flowing fluid on the transducer surfaces adversely alters the transmission and reception characteristics thereof, thereby further adding inaccuracies to the fluid velocity measurements obtained. Efforts to overcome these and other disadvantages of known flowmeter systems of the above type have not met with wide success.